Several years ago, Mary Oliver became that rare paradox, a bestselling poet. Although Americans buy and read very little poetry, Oliver’s slender collections, such as “Dog Songs” and “A Thousand Mornings,” have enjoyed popular appeal. Part of the reason, one gathers, is that Oliver writes poems about nature that are simple and easy to understand – or at least appear to be at first glance. But like Robert Frost, who was also praised for poems about the outdoors that seemed plain and straightforward, Oliver’s verse reveals, upon closer scrutiny, deeper and more complex themes.

The same can be said for Upstream, which collects Oliver’s best previously published essays and includes one new piece, “Provincetown,” a homage to the seaside Massachusetts community where she lived and worked for 50 years. Now 81 and without longtime partner Molly Mallone Cook, a celebrated photographer who died in 2005, Oliver currently lives in coastal Florida. But for Oliver, a native of Ohio who took to the woods early to escape a troubled youth, the landscape and literature of New England has defined her vision. “Upstream” includes an essay about Ralph Waldo Emerson, who championed the transcendental idea of a direct relation with God through nature. Oliver writes that transcendentalism “is hardly a proper philosophy,” so loosely constructed that it can mean many things to many people. Even so, in the Emersonian tradition, she finds the most durable connections with the divine in the woods, a bond forged more through intuition than formal logic.

“Knowledge has entertained me and it has shaped me and it has failed me,” she writes in another essay here, “Winter Hours.” “Something in me still starves. In what is probably the most serious inquiry of my life, I have begun to look past reason, past the provable, in other directions. Now I think there is only one subject worth my attention and that is the precognition of the spiritual side of the world and, within this recognition, the condition of my own spiritual state. I am not talking about having faith necessarily, although one hopes to. What I mean by spirituality is not theology, but attitude.”

Like another transcendentalist, Henry David Thoreau, who famously did a lot of traveling in Concord, Oliver found a universe of meaning close to home, in the woods of Provincetown. “I have never been to Rome. I have never been to Paris, or Greece, or Sweden.... I am not a traveler. Not of that sort,” she confesses.

For a half a century, among the trees shading Provincetown, Oliver managed to find something new, or to see something familiar with new eyes. “Through these woods I have walked thousands of times,” she writes. “For many years I felt more at home here than anywhere else, including our own house. Stepping out into the world, into the grass, onto the path, was always a kind of relief. I was not escaping anything. I was returning to the arena of delight.”

Oliver does find delight in nature, but more vexing realities, too. In “Bird,” one of the book’s best essays, Oliver recalls the Christmas morning that she took an injured gull home from the beach. She and Cook made a pet of the gull and did everything they could to prolong its life, although there was no hope for a full recovery. Unsure of their choice, they attempted to reverse course, releasing the crippled creature back to the wild, then retrieving the bird when it became obvious the gull would quickly die on its own. The experience raised ethical quandaries. Was bringing a wild thing within a household a form of compassion or conceit? Was it right to extend the life – and perhaps pain – of a dying bird? To what degree do we revere nature not by embracing it, but keeping it at arm’s length, acknowledging the distance that divides us from the world, the gulf of mystery between man and planet?

Oliver hints at these questions, though she doesn’t attempt answers. At the end of “Bird,” the gull inevitably expires, Oliver sharing the news “as I lifted the shades to the morning light.”

It’s a subtle reminder that life goes on for the rest of the world even after we leave it. The message carries special resonance in “Upstream,” which conveys a sense of valediction for a writer in the final phase of her life and work.

Asked what she had done with her life, Oliver once answered succinctly: “Used a lot of pencils.”